You can’t delete replies, but you can edit them.
At the bottom of your post, you should see a little grey pencil in a row with some other icons. Tap on the grey pencil and in the post you want to edit, and it will come up in a dialogue box. Edit it as you want to. You can delete the whole post and write “deleted”. Or you can add or take away things. Then hit save edit, and the edited version will appear.
There is a window of time in which you can do this. If you wait too long, you can’t edit.